Problem: Ben is 4 times as old as Luis and is also 18 years older than Luis. How old is Ben?
Answer: We can use the given information to write down two equations that describe the ages of Ben and Luis. Let Ben's current age be $b$ and Luis's current age be $l$ $b = 4l$ $b = l + 18$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $b$ is to solve the second equation for $l$ and substitute that value into the first equation. Solving our second equation for $l$ , we get: $l = b - 18$ . Substituting this into our first equation, we get the equation: $b = 4$ $(b - 18)$ which combines the information about $b$ from both of our original equations. Simplifying the right side of this equation, we get: $b = 4b - 72$ Solving for $b$ , we get: $3 b = 72$ $b = 24$.